Polyimides

ABSTRACT

Terpolyimides prepared from tetracarboxylic acid esters and combinations of heterocyclic, aromatic, and aliphatic diamines and artifacts composed of those terpolymers. Precursors, and methods of converting them to the corresponding terpolymers.

The invention described herein was made in the performance of work underNASA Contract No. NAS9-15484 and is subject to the provisions of Section305 of the National Aeronautics and Space Act of 1958 (72 Stat. 435; 42USC 2457).

This application is a division of application Ser. No. 254,137 filedApr. 14, 1981, 1981 Application Ser. No. 254,137 is acontinuation-in-part of Application Ser. No. 186,668 filed Sept. 12,1980 (now abandoned).

The present invention relates, in one aspect, to polyimides and, moreparticularly, to certain novel polyimides which have improved propertiesby virtue of their being terpolymers derived from tetracarboxylic acidsand combinations of heterocyclic, aromatic, and aliphatic diamines.

In other aspects our invention relates to precursors of the just alludedto terpolyimides and their preparation and to the conversion of theprecursors to the corresponding terpolymers.

U.S. Pat. No. 30,213 issued Feb. 12, 1980, to John Gagliani et al forMETHOD OF MAKING FOAMED COPOLYIMIDES AND PRODUCTS OBTAINED THEREFROM andcopending U.S. patent application Ser. No. 935,378 filed Aug. 21, 1978,by John Gagliani for POLYIMIDES disclose hydrolytically stablecopolyamide foams which are fire resistant and give off essentially nosmoke or toxic fumes when they are heated to degradation temperatures(application Ser. No. 935,378 was copending with application Ser. No.186,668 which is the parent of application Ser. No. 254,237).Consequently, those foams are useful in aircraft cabins, space vehicles,and land and sea transport and in a variety of other applications wherehuman life or equipment might be endangered by the overheating ofconventional, more flammable, smoke-emitting materials. They can beused, in such applications, in fire containing walls and lightweightstructures, to protect fuel tanks and heat sensitive systems, and asvoid filler materials and thermal, cryogenic, thermal, electrical andacoustical insulations, for example.

We have now discovered that polyimide foams which are in many wassuperior to those identified above can be made without sacrificing thedesirable attributes of the latter by adding a third diamine ofaliphatic character to the precursor from which the polyimide is made.Typical advantages of such foams are increased flexibility andresiliency, greater fatigue resistance, and improved compression setproperties.

Compression set is a measure of the extent to which a foam will take ona permanent set or deformation after having been compressed to a statedfraction of its original thickness for a prolonged period of time. Thisis important in seating applications, for example; materials which aresusceptible to compression set reach the point where tactile comfortbecomes unacceptable much sooner than those having good compression setproperties.

Generally speaking, then, our novel terpolyimides disclosed herein areprepared from precursors which are solutions of a lower alkyl ester of3,3',4,4'-benzophenonetetracarboxylic acid or a mixture of such esters,an aromatic diamine which is free of aliphatic moieties, a heterocyclicdiamine, and an aliphatic diamine. The imide-forming functionalities(the amino and carboxylic moieties) are preferably present insubstantially equimolar amounts.

Exemplary of the aromatic and heterocyclic diamines that can be employedare:

2,6-diaminopyridine

4,4'-diaminodiphenyl sulfone

p,p'-methylene dianiline

4,4'-diaminodiphenyl ether

Many other aromatic and heterocyclic diamines have been described in theopen and patent literature dealing with the making of polyimides--see,for example, U.S. Pat. Nos. 3,179,614 issued Apr. 20, 1965, to Edwardsfor POLYAMIDE-ACIDS, COMPOSITIONS THEREOF AND PROCESS FOR THEIRPREPARATION; U.S. Pat. No. 3,575,891 issued Apr. 20, 1971, to LeBlanc etal. for STABILIZED POLYIMIDES; and U.S. Pat. No. 3,629,180 issued Dec.21, 1971, to Yoda et al for PROCESS FOR THE PRODUCTION OF A THERMALLYSTABLE POLYMER. Aromatic and heterocyclic diamines selected from thoselisted in the foregoing patents can be utilized in terpolyimides inaccord with the principles of our invention as can others; and weconsequently consider our invention to embrace the use of all operablearomatic and heterocyclic diamines.

Aliphatic diamines having from three to 12 carbon atoms have beenemployed; however, diamines having no more than six carbon atoms willtypically prove preferable. The use of those with longer chains can leadto excess thermoplasticity, and that can cause the foam to collapse asit is generated. Also, aliphatic diamines with even numbered chains arepreferably employed as they are capable of imparting greater thermalstability to terpolyimides of the character described herein thanaliphatic diamines with odd numbered chains.

Aliphatic diamines we have employed include:

1,3-diaminopropane

1,4-diaminobutane

1,6-diaminohexane

1,8-diaminooctane

1,12-diaminododecane

and Jeffamine 230. The latter is available from the Jefferson ChemicalCompany and has the formula: ##STR1## where x is approximately 2.6.

From 0.05 mole of aliphatic diamine per 1.0 mole ofbenzophenonetetracarboxylic acid can be employed. Concentrations of notmore than 0.2 mole per mole of acid are, however, preferred inapplications where inflammability is a requirement as the flameresistance of our terpolymers decreases considerably at higherconcentrations of the aliphatic diamine.

From 0.05 to 0.9 mole of heterocyclic diamine per mole of acid can beused. Terpolymers with the higher concentration of heterocyclic diaminehave the best compression set values and are therefore favored in seatcushioning and other applications of our invention where that propertyis important.

The precursors of our terpolyimides are essentially monomeric, liquid orsolid state solutions of the selected ester (or esters) and diamines.

They are prepared by first reacting3,3',4,4'-benzophenonetetracarboxylic acid or, preferably, itsdianhydride with an esterification agent to form an alkyl diester.Exemplary esterification agents are methyl, ethyl, propyl, and isopropylalcohols. Methanol is in many cases preferred because of its widespreadavailability, low cost, and other attributes; because its usefacilitates conversion of the precursor to a polyimide foam; and becausethe foams made from the methyl esters tend to be more flexible,resilient, and compression set resistant. Ethanol is also a preferredesterification agent.

The esterification reaction is followed by the addition of the diamines,which are dissolved in the reaction mixture. The temperature is keptbelow the reflux temperature of the esterification agent duringdissolution of the diamines and low enough to avoid polymerization ofthe diamines and ester.

Graphite, glass, and other fibers, as well as other fillers such asglass microballoons and additives such as crosslinking agents can beadded to the resulting composition to impart wanted properties to thefinal product. A surfactant can also be added to increase fatigueresistance of the terpolyimide foam and to make it more flexible andresilient by increasing the bubble stability of the foam and theuniformity of the cellular structure.

One preferred surfactant is AS-2, a nonionic, fluorinated, polyalkylenecopolymer manufactured by E. I. DuPont de Nemours and Company. We haveemployed from 0.01 to 0.1 percent of this surfactant based on the weightof the ester and diamine constituents. In systems containing 2,6-diaminopyridine and p,p'-methylene dianiline along with the aliphatic diamineand 3,3',4,4'-benzophenonetetracarboxylic acid ester, a concentration ofca. 0.05 percent proved to be optimum.

Another surfactant that has been successfully employed in those systemsin concentrations of 0.1 percent is X-3, a nonionic surfactant of thesame general chemical composition as AS-2 and manufactured by the samecompany.

The material existing after dissolution of the diamines and the additionof any additives may range in form from a "liquid resin" to aspreadable, pastelike formulation depending upon the nature and quantityof any fillers added to the resin. The material may be used in the formjust described; or it can be transformed into an amorphous powdercapable of being converted into a flexible, resilient, terpolyimidefoam. Although not essential, it is preferred that spray drying beemployed for this purpose because the liquid resin can thereby betransformed on a continuous basis and in one step into a dry powder.Also, spray drying allows for modification of the precursor in wayswhich can be used to vary the properties of the final product.

One suitable spray drying process is described in copending applicationSer. No. 186,670 filed Sept. 12, 1980.

The amphorous, powdered resinoid precursor can be converted to amonolithic, terpolyimide foam by various techniques includingdielectric, thermal, and microwave heating. The latter, alone or with athermal post-cure, is preferred because of the speed with which the foamcan be generated and cured; because the foam is homogeneously heated;and because handling of the fragile, uncured foam can be avoided.

Microwave techniques and equipment that can be used to foam and cure theprecursor are disclosed in copending application Ser. No. 186,629 filedSept. 12, 1980.

Foaming-curing parameters that have proven satisfactory in converting100 gram samples of representative precursors to flexible, resilientterpolyimide foams are two to 12 minutes exposure to high frequencyradiation in an oven operating at a frequency of 2450 MHZ and at 5 kWpower followed by thermal heating at a temperature of 500°-550° F. for15 minutes to two hours.

The resulting foam can be employed as such--in a seat cushion or asinsulation, for example. Or, using the procedure described inapplication No. 935,378 as a further example, the flexible, resilientterpolyimide foam can be converted to a dense, rigid, structurallystrong, intumescent material by heating it under pressure.

As suggested above, there are also applications in which the precursorcan best be utilized in a liquid or semifluid form. One example is themaking of wall and floor panels and other rigid components or artifacts.In a typical application of that character, a layer of the liquid resin,compounded with appropriate fillers, is sandwiched between two pieces ofglass cloth wetted with the resin. Foaming and curing of theterpolyimide in a typical wet panel thus formed can be effected in muchthe same manner as the powdered precursors.

The general model for the chemical reactions which are effected informing the precursor and in converting it to a polyimide are shownbelow. The actual reactions are much more complex as three amines,rather than the single aromatic amine shown in the model, are involved##STR2##

One advantage of our invention, alluded to above, is that advantage canbe taken of its principles to provide polyimides which have theattributes of those state-of-the-art copolyimides disclosed in U.S. Pat.No. 30,213 and in application Ser. No. 935,378, and, at the same time,have such additional desirable attributes as a wider range of mechanicalproperties, greater flexibility and resilience, and greater fatigueresistance and durability.

Also, our novel compositions have the advantage of great versatility;they can, for example, be produced as foams useful for cushioning and inother applications where comfort is important, and as thermal,electrical, and acoustical insulations; and they can, on the other hand,be used in floor and wall panels and in other rigid components. They canalso be molded into a wide variety of configurations; and fillers andother additives can be compatibly compounded with them to provideoptimal performance in various applications of our invention.

Terpolyimides as described above are unique insofar as we are aware.U.S. Pat. Nos. 3,573,132 issued Mar. 30, 1971, to Ducloux et al. forPROCESS FOR THE PREPARATION OF POLYIMIDES and U.S. Pat. No. 4,043,978issued Aug. 23, 1977, to Schmidt et al. for POLYIMIDES suggest thatpolyimides can be made from tetracarboxylic acids or anhydrides andaliphatic, aromatic, heterocyclic and other diamines and that mixturesof diamines can be employed. However, there is nothing in either patentwhich would lead one to the particular combination of aromatic plusheterocyclic and aliphatic diamines we employ or which suggests therelative proportions of diamines needed to take advantage of thatcombination; and there is nothing in either patent which suggests how apolymer with our particular combinations of diamines could be made orthat anything would be gained by making such compositions.

From the foregoing it will be apparent to the reader that one importantand primary object of our invention resides in the provision of a newfamily of polymers which are unique in a variety of respects.

Related and also important but more specific objects of the presentinvention include:

the provision of polymers and compositions containing them which arefire resistant and which give off little or no smoke or toxic compoundswhen subjected to high temperature, oxidative degradation.

which are strong and durable;

which, when formulated as foams, have improved flexibility andresiliency, fatigue resistance, and compression set;

which are so versatile that they can be employed for such diversepurposes as cushioning and insulation of different types, in panels andother components requiring rigidity and structural strength, and asmolding powders.

Another important and again related object of our invention resides inthe provision of novel, improved polymers which are terpolyimidesderived from a benzophenonetetracarboxylic acid ester and a combinationof aromatic, heterocyclic, and aliphatic diamines.

Still other important and primary objects of the present inventionreside in the provision of precursors for the polymers identified aboveand in the provision of processes for making those polymers and forconverting the precursors to the corresponding polymers.

Certain important objects of the present invention have been identifiedabove. Other important objects and advantages and additional novelfeatures of the invention will be apparent to those skilled in therelevant arts from the foregoing general description of the invention;from the appended claims; and from the following examples, which areintended to illustrate and not restrict the scope of the invention.

EXAMPLE I

3,3',4,4'-Benzophenonetetracarboxylic acid dianhydride (BTDA) (322.23 g,1.0 mole) was added to 240 ml of methyl alcohol and 24 ml of water in aone-liter, three-neck flask equipped with a thermometer, a mechanicalstirrer, and a reflux condenser. After addition, the mixture wasrefluxed until clear. The mixture was then refluxed for an additional 60minutes to ensure complete reaction of the BTDA to its half (or-di-)ester.

The contents of the flask were then cooled to 25°-35° C. (77°-95° F.).

2,6-Diaminopyridine (2,6 DAP) (32.8 g, 0.3 mole) and p,p'-methylenedianiline (MDA) (99.1 g, 0.5 mole) were added to the half ester solutionand the contents mixed for 15 minutes.

1,6-Diaminohexane (1,6 DAH) (23.7 g, 0.2 mole) was next added to themixture. This was done slowly enough that the reaction temperature didnot exceed 65° C. (149° F.).

The result was a liquid resin precursor which can be used in that formas discussed above.

In a typical instance involving the formation of a low density, highstrength, rigid panel, the liquid resin is compounded with selectedfillers in a variable speed mixer until the fillers are thoroughlywetted. Glass cloth wetted with the resin is placed on a sheet ofaluminum foil. The resin mixture is spread over the glass cloth andcovered with another piece of liquid resin wetted glass cloth. Solventis removed by drying the wet panel in a microwave oven on a sheet ofTeflon coated glass cloth at a power output of 1.25 KW for a period of 3to 5 minutes.

The dried panel is then foamed and cured. Foaming of the panel can becarried out in the microwave oven at a power output of 5.0 KW for sixminutes between two sheets of Pyroceran with the thickness of the panelbeing controlled by Teflon spacers extending between the sheets. Thepanels can then be cured in a circulating air oven at a temperature of287.7° C. (550° F.) for 30 minutes.

EXAMPLE II

In an instance leading to a flexible, resilient foam, a liquid resin asdescribed in Example I and made by the process described in that Examplewas first compounded with 0.1 weight percent of X-3 surfactant, based onthe weight of its ester and amino constituents, and then mixed with a 30phr (parts per hundred parts of resin) dilution ratio of alcohol. A NiroMobile spray dryer was heated to an inlet temperature of 100° C. (212°F.) and an outlet temperature of 70° C. (158° F.). The liquid resin wasthen fed into the dryer with the feed being manually adjusted throughoutthe operation to keep the dryer outlet temperature in the range of69°-71° C. (156°-160° F.).

This produced a powder which was collected, sieved through a Tyler 48mesh (297 microns) sieve, and rolled for 30 minutes in a round plasticbottle.

This powder is, essentially, a solid state solution of unreacteddiamines and 3,3',4,4'-benzophenonetetracarboxylic acid diester.

A flexible terpolyimide insulating foam was produced from the powderprecursor using a Gerling Moore Batch Cavity Model 4115 microwave ovenpreparing at a frequency of 2450 MHz and a power of 5 KW.

The precursor was spread on a Teflon coated glass cloth substrate andplaced in the microwave cavity at room temperature. After two to twelveminutes of exposure to the microwave field, depending upon theparticular test being conducted, the powder expanded into a homogeneous,cellular foam block. This block was thermally cured into a flexible andresilient foam by heating it at 260° C. (500° F.) for two hours.

The foam rise, cellular structure, resiliency, density, fatigueresistance, and compression set of the foam were then identified.

Resiliency was determined by the ball rebound method described in ASTMDesignation D-1564, Suffix B, using a tester fabricated and calibratedin accord with that procedure.

Compression set of the foam at 90 percent compression was determinedaccording to the same ASTM Designation, Method B, using two steel platesheld parallel to each other by clamps. The space between the plates wasadjusted to the required thickness by spacers.

The resistance of the foam to cycle shear loadings; i.e., its fatigueresistance, was determined in accord with ASTM Designation D-1564,Procedure B, with the exception that examination and measurement of thefoam for loss of thickness was made at 10,000 and 20,000 cycles. Thefatigue tester was constructed in accord with the same ASTM Designation.

Performance of the foam was detected qualitatively by looking forembrittlement and degradation of the cellular structure andquantitatively by the ball rebound resiliency method and by weightchange.

Other tests involved visual observation of the products for cellularstructure, foaming behavior, and imperfections; flame resistance using aMeker burner; and hydrolytic stability.

Numerical and qualitative test results are tabulated below:

                  TABLE 1                                                         ______________________________________                                                 90% Compression                                                               Set                                                                  Density  % Loss After Resiliency                                              lbs/         30 Minute    Ball    Foam                                        ft.sup.3                                                                           kg/m.sup.3                                                                            Recovery     Rebound Characteristics                             ______________________________________                                        1.44 23.0    30           55      Flexible, resilient,                                                          medium cell size                            ______________________________________                                    

The foam resisted the open flame of the Meker burner for up to 20minutes, and it exhibited almost no change after having been kept at 100percent relative humidity at 60° C. (140° F.) for 30 days.

EXAMPLE III

To demonstrate that other aliphatic diamines can be employed in thenovel family of polymers disclosed herein, the procedure described inExample I was repeated, using a variety of aliphatic diamines. Theliquid resins thus obtained were then dried and converted toterpolyimide foams using the procedure described in Example II.

Also, to further illustrate how modifications in the formulation of theprecursor can be utilized to control the properties of the terpolyimide,the molar ratios of the aliphatic and heterocyclic diamines to the BTDAester were varied over a considerable range.

The resulting terpolyimide foams were subjected to the analysesdescribed in Example II. Many of the results are tabulated below, andothers are discussed in the narrative following the Table:

                                      TABLE 2                                     __________________________________________________________________________                                90% Compression Set                                                                      Resiliency                             Foam Resin                                                                            (Molar Ratios)                                                                           Density  % Loss After                                                                             Ball                                   Number.sup.3                                                                          Aliphatic Diamine.sup.1                                                                  lbs/ft.sup.3                                                                       kg/m.sup.3                                                                        30 Minute Recovery                                                                       Rebound                                                                             Foam Characteristics             __________________________________________________________________________    Copolyimides                                                                          None       0.538                                                                               8.6                                                                              52         55    Flexible, resilient, good                                                     structure                        Group 1 (1.0:0.3:0.6:0.1)                                                     1       Propyl.sup.2                                                                             1.44 23.0                                                                              46         50    Flexible, resilient, good                                                     structure                        2       Butyl      1.32 21.1                                                                              63         45    Flexible, resilient, good                                                     structure                        3       Hexa       1.36 21.8                                                                              48         55    Flexible, resilient, good                                                     structure                        4       Octa       0.943                                                                              15.1                                                                              39         50    Flexible, resilient,                                                          striated                         5       Dodeca     1.62 25.9                                                                              42         50    Flexible, resilient, large                                                    cell                             6       Jeffamine D-230                                                                          1.11 17.8                                                                              21         70    size, brittle                    Group 2 (1.0:0.2:0.6:0.2)                                                     7       Propyl     0.840                                                                              13.4                                                                              40         40    Flexible, resilient, good                                                     structure                        8       Butyl      1.25 20.0                                                                              53         53    Flexible, resilient, good                                                     structure                        9       Hexa       0.817                                                                              13.1                                                                              47         55    Flexible, resilient, good                                                     structure                        10      Octa       1.40 22.4                                                                              43         35    Flexible, resilient, good                                                     structure                        11      Dodeca     3.32 53.0                                                                              46         70    Flexible, resilient, good                                                     structure                        12      Jeffamine D-230                                                                          --   --  --         --    Brittle, very large cell                                                      size, poor                                                                    foam                             Group 3 (1.0:0.1:0.6:0.3)                                                     13      Propyl     --   --  --         --    Rigid foam, collapsed and                                                     degraded                                                                      on heating                       14      Butyl      1.48 23.7                                                                              63         50    Flexible, resilient, fair                                                     structure                        15      Hexa       1.37 21.9                                                                              71         50    Flexible, resilient, fair                                                     structure                        16      Octa       1.33 21.2                                                                              68         45    Flexible, resilient, good                                                     structure                        17      Dodeca     0.778                                                                              13.5                                                                              45         70    Flexible, resilient, good                                                     structure                        Group 4 (1.0:0.3:0.5:0.2)                                                     18      Propyl     1.33 21.2                                                                              40         50    Flexible, resilient, good                                                     structure                        19      Butyl      0.835                                                                              13.4                                                                              25         45    Flexible, resilient, good                                                     structure                        20      Octa       0.845                                                                              13.5                                                                              22         70    Flexible, resilient, medium                                                   cell size                        21      Hexa       1.44 23.0                                                                              30         55    Flexible, resilient, medium                                                   cell size                        22      Dodeca     0.565                                                                               9.04                                                                             23         65    Flexible, resilient, good                                                     structure                        23      Jeffamine D-230                                                                          --   --  --         --    Brittle, very large cell                                                      size,                                                                         collapsed on heating             Group 5 (1.0:0.3:0.4:0.3)                                                     24      Butyl      1.15 18.3                                                                              31         50    Flexible, resilient, good                                                     structure                        25      Hexa       0.399                                                                               6.36                                                                              7         55    Flexible, resilient, highly                                                   reticulated                      Group 6 (1.0:0.3:0.55:0.15)                                                   26      Hexa       1.17 18.7                                                                              44         --    Flexible, resilient, good                                                     structure,                                                                    voids                            Group 7 (1.0:0.3:0.65:0.05)                                                   27      Hexa       1.08 17.3                                                                              31         --    Flexible, resilient, good                                                     structure,                                                                    voids                            __________________________________________________________________________     .sup.1 In the order of: 3,3' ,4,4benzophenonetetracarboxylic acid ester;      2,6diamino pyridine; p,pmethylene dianiline; aliphatic diamine.               .sup.2 Indication of a radical is used to identify the corresponding          aliphatic diamine; e.g., "propyl" = 1,3diamino propane.                       .sup.3 Each resin contained 0.1 weight percent of X3 surfactant, and          methanol was used as the esterification agent.                                .sup.4 This entry, provided for comparison purposes, involved a               copolyimide foam derived in essentially the same manner as the foams of       Groups 1-7 from a precursor having a 1.0:0.3:0.7 molar ratio of 3,3'          ,4,4benzophenonetetracarboxylic acid ester; 2,6diamino pyridine; and          p,pmethylene dianiline.                                                  

The Group 1 foams generally exhibited better compression set values andhigher density than foams produced from the copolyimide resin with theJeffamine D-230 giving useable foams with excellent compression setvalues (when these foams were scaled up to a large size, the quality ofthe cell structure worsened).

The Group 2 foams were generally comparable in mechanicalcharacteristics to those of Group 1 except that Jeffamine 230 producedfoams which exhibited poor characteristics.

Foams of Group 3 produced smoke and continued to burn for 5-15 secondsafter removal from the flame. However, these foams were in many respectssatisfactory; and they can accordingly be used where flame resistance isnot a controlling criteria.

The Group 4 foams had the most homogeneous cellular structure with theexception of foams made with Jeffamine D-230 (Resin 22). A significantadvantage of the foams derived from the precursors of Group 4 is theimproved compression set.

The densities of these foams are considerably higher than those of thecopolyimide foam which is a drawback in applications where weight is ata premium. However, this is offset by increased fatigue resistance.

The Group 5 foams had good structure and excellent compression setproperties in one case. However, these foams were found to be less fireresistant than is characteristic of polyimides.

The Group 6 and 7 foams show the effect of varying the concentration ofthe preferred aliphatic diamine (1,6-diamino hexane). The No. 26 foamwas of particular interest. It had considerably decreased fireresistance, indicating that aliphatic diamines concentrations lower than0.3 mole should be employed in applications where maximum fireresistance is wanted, at least if the aliphatic amine is 1,6diaminohexane.

EXAMPLE IV

That the properties of our novel terpolyimides can be selectivelyaltered by incorporating a surfactant in the precursor and by varyingits concentration is shown by a series of tests conducted as describedin Example I and II except that AS-2 surfactant was employed. Thesurfactant concentration of the 1.0 BTDA ester; 0.3 2,6 DAP:0.5 MDA:0.2DAH formulation was varied from 0.1 to 1.5 weight percent. The resultsare tabulated below:

                                      TABLE 3                                     __________________________________________________________________________    Surfactant           Indentation Load             After Fatigue               (AS-2)               Deflection (ILD).sup.1                                                                        Compression                                                                          Resiliency                                                                          (10,000 Cycles)             Foam Concentration                                                                         Density N       Lbs     Set Loss                                                                             Before      Height Loss           No.  Percent Kg/m.sup.3                                                                        Lbs/ft.sup.3                                                                      25% 65% 25% 65% (Percent)                                                                            Fatigue                                                                             Resiliency                                                                          Percent               __________________________________________________________________________    1    0.1     24.0                                                                              1.5 293.6                                                                             1427.8                                                                            66  321 49     50    55    +3.9                  2    0.25    25.6                                                                              1.6 266.9                                                                             1352.2                                                                            60  304 37     45    47    +5.3                  3    0.5     22.4                                                                              1.4 266.9                                                                             1165.4                                                                            60  262 35     40    37    +2.0                  4    0.75    22.4                                                                              1.4 195.7                                                                             1009.7                                                                            44  227 34     40    43    -2.9                  5    1.0     18.4                                                                              1.15                                                                              155.7                                                                              800.7                                                                            35  180 27     45    *     *                     6    1.5     17.0                                                                              1.06                                                                              155.7                                                                              809.5                                                                            35  182 27     50    *     *                     __________________________________________________________________________     *Cellular structure collapsed after fatigue                                   .sup.1 Performed in accord with ASTM Standard D1564                      

EXAMPLE V

We pointed out above that the order in which the diamines are added tothe solution of esterified BTDA is an important feature of ourinvention. This is demonstrated by tests in which the procedures ofExamples I and II were followed except for use of the Resin 21formulation and:

(a) addition of the heterocyclic diamine followed in order by thearomatic and aliphatic diamines at intervals of 15 minutes; and

(b) addition of the aromatic diamine followed in order by theheterocyclic and aliphatic diamines at intervals of 15 minutes.

The addition of the diamines was started with the reaction mixture at atemperature of 30°-35° C. (86°-95° F.). The temperature was allowed toincrease freely to approximately 50° C. (122° F.) and then controlled byreducing the rate of the addition of the diamines. Finally, the reactionmixture was heated to and maintained at 60°-65° C. (140°-149° F.) forfive minutes.

Addition and complete dissolution of the aromatic diamine before theheterocyclic and the aliphatic diamines were added (option b) producedfoams with fewer flaws and significantly less foam collapse at the endof the curing cycle.

EXAMPLE VI

Particle size is another parameter that significantly affects theproperties of terpolyimide foams prepared in accord with the principlesof our invention. This was demonstrated by a series of tests involving aterpolyimide containing diaminohexane.

Various particle sizes were obtained by sieving the powdered precursorthrough a Tyler mesh screen and by comminuting it in a Pulvette benchmodel grinder.

All foams were produced by foaming and curing the precursor on Tefloncoated glass (type 7267/114) in a 15 kW microwave oven using a powderloading of 15 kg (33 lbs.) at a thickness of 6.35 cm (2.5 inches).

The data resulting from this series of tests is summarized below:

                  TABLE 4                                                         ______________________________________                                                 Indentation Load                                                              Deflection                                                           Particle Size                                                                            25%        65%                                                     (Tyler Mesh)                                                                             N      (lbf)   N    (lbf) Foam Quality                             ______________________________________                                        #25        138    39       534 120   Good cellular                                                                 structure                                #50        245    55      1076 242   Rigid structure                          Pulverized 267    60      1054 237   Rigid structure,                         (maximum size                        large flaws                              less than 50                                                                  microns)                                                                      ______________________________________                                    

The tabulated data suggests that larger particle size precursors yieldmore usable foams and a more homogeneous cellular structure with fewerimperfections than foams with a maximum particle size of 50 microns andlower.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The presentembodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription; and all changes which come within the meaning andequivalency of the claims are therefore intended to be embraced therein.

What is claimed and desired to be secured by United States LettersPatent is:
 1. A foamable terpolyimide precursor comprising anessentially stoichiometric mixture of an alkyl ester of3,3',4,4'-benzophenonetetracarboxylic acid or a mixture of such estersand at least three diamines, one of said diamines being heterocyclic, asecond of said diamines being aromatic, a third of said diamines beingaliphatic, and said precursor containing from 0.05 to 0.5 mole ofaliphatic diamine and from 0.05 to 0.9 mole of heterocyclic diamine permole of benzophenonetetracarboxylic acid ester(s).
 2. A terpolyimideprecursor as defined in claim 1 in which the aliphatic diamine has fromthree to twelve carbon atoms.
 3. A terpolyimide precursor as defined inclaim 2 in which the aliphatic diamine has not more than six carbonatoms.
 4. A terpolyimide precursor as defined in either of the precedingclaims 2 or 3 in which the aliphatic amine has an even number of carbonatoms.
 5. A terpolyimide precursor as defined in claim 1 in which thealiphatic diamine has the formula: ##STR3## where x is on the order of2.6.
 6. A terpolyimide precursor as defined in claim 1 which includes asurface active agent in an amount effective to improve the bubblestability of and produce a more uniform cellular structure in thepolyimide to which the precursor is converted.
 7. A terpolyimideprecursor as defined in claim 1 which is a dry powder and which has amaximum particle size of at least 50 microns.
 8. A terpolyimideprecursor as defined in claim 1 in which the ester constituent is amethyl or ethyl half ester of 3,3',4,4'-benzophenonetetracarboxylic acidand the diamines are 2,6-diamino pyridine, p,p'-methylene dianiline, and1,6-diamino hexane.
 9. A terpolyimide precursor as defined in claim 8 inwhich the molar ratio of said constituents is:

    ______________________________________                                        3,3',4,4'-benzophenonetetracarboxylic acid                                                              1.0                                                 2,6-diamino pyridine      0.3                                                 p,p'-methylene dianiline  0.4-0.65                                            1,6-diamino hexane        0.05-0.3.                                           ______________________________________                                    


10. A terpolyimide precursor as defined in claim 1 which includes afiller intimately admixed with said mixture of diamines andtetracarboxylic acid ester(s).